The memo also offered a defence in case any interrogator was charged with breaking US or international laws.

"Finally, even if the criminal prohibitions outlined above applied, and an interrogation method might violate those prohibitions, necessity or self-defence could provide justifications for any criminal liability," the memo concluded.

The Pentagon, unlike US intelligence bodies, specifically prohibited its interrogators from using certain harsh methods, including a simulated drowning technique known as waterboarding.

The memo was rescinded in December 2003, nine months after Yoo sent it to the Pentagon's top lawyer, William J Haynes.

Democratic criticism

There have been widespread protests overUS prisoner abuse scandals [Reuters]

The US has faced widespread criticism over a series of prisoner abuse scandals at Abu Ghraib prison in Iraq in 2003 and the Guantanamo Bay detention centre.

"Senior officials at the justice department gave the Pentagon the green light to torture prisoners,"Amrit Singh, an ACLU lawyer, said.

Singh said the memo, and a similar 2002 opinion for the CIA, undermine the Bush administration's argument that abuses such as Abu Ghraib in 2003 were aberrations.

"These memos just go to show that it was the policies of the Bush administration that was driving this abuse," she said.

Democratic party politicians also condemned the memo and called for those who approved it to be held accountable.

"This memo created the lawless atmosphere that led directly to the abuse of detainees at Abu Ghraib. Those who wrote it and those who approved it should be held accountable," Joseph Biden, a Democratic senator, said.

The memo by Yoo also included past legal defences of other interrogation methods that were not considered torture, such as sleep deprivation, hooding detainees and "frog crouching,'' which forces prisoners to crouch while standing on the tips of their toes.

The "definition of torture must be read as a sum of these component parts," the memo said.

"Our policy is to treat detainees humanely and that has always been the case," Commander JD Gordon, a Pentagon spokesman, said.